In an SOFC, a layer of cathode material is provided on a first side of a layer of solid oxide electrolyte material and a layer of anode material is provided on an opposite, second side of the solid oxide electrolyte material. YSZ is one form of solid oxide electrolyte used for this purpose. However, deleterious reactions can occur between the cathode material and the YSZ electrolyte material that result in the formation of electrically insulating phases between the two. These insulating phases can have an impact on the stability and power density of the SOFC. For example, with the use of high performance cathode materials comprising lanthanum and strontium, the insulating phases that can form comprise oxides of lanthanum and zirconium and/or strontium and zirconium, including LaZrO3 and SrZrO3.
The insulating phases can form during use of the SOFC, but the problem primarily arises during manufacture of the SOFC, especially under the high temperature conditions required to prepare anode-supported SOFCs as discussed below.
To alleviate the formation of the insulating phases, it has been proposed to provide a reaction barrier layer between the layer of cathode material and the layer of YSZ electrolyte material, to prevent or at least decrease formation of the insulating phases. The reaction barrier layer is applied during manufacture of the SOFC, before the application of the cathode material to the electrolyte material. The electrolyte material having the reaction barrier layer precursor materials on the first side of the electrolyte layer can be sintered together, optionally with the anode precursor materials on the second side, at a suitable temperature, for example in the range of from about 1200° C. to 1500° C. The layer of cathode material can then be applied on to the reaction barrier layer as formed and the components sintered again, usually at a lower temperature, to provide the SOFC.
A proposed suitable reaction barrier layer comprises doped ceria CeO2 (doped with either yttria (Y2O3), samaria (Sm2O3) or gadolinia (Gd2O3)), because there are no deleterious reactions between the doped ceria and the cathode. However, ceria is difficult to sinter into a coherent tough film at the elevated temperatures used, because oxygen evolves from the lattice and ceria and the dopant ions diffuse into the YSZ electrolyte leaving behind voids in the ceria reaction barrier layer. These internal voids in the reaction barrier layer, which can be in the range of from about 5 to 10 microns thick, lead to poor cell performance. Accordingly, the reaction barrier layer causes the SOFC to be unreliable.
One way of minimising the problem of ceria migration is to apply the ceria after the anode and the YSZ electrolyte have been co-fired together at high temperatures (in the range of from about 1200° C. to 1500° C.). The applied doped ceria can then be sintered, on the layer of electrolyte material, at a lower temperature in an extra firing step. However, an extra firing step is undesirable, because of the associated higher cost of SOFC manufacture.
Accordingly, there exists a need for a reaction barrier layer that performs better than that described above, and for a method of providing the reaction barrier layer on to the YSZ electrolyte.